The present invention relates to a heater control device, and more particularly to an improved heater control device in which a heater doubles as a temperature sensor for the purpose of eliminating the leading wires to be used exclusively for a temperature sensor.
There is shown in FIG. 1 a circuit diagram of a conventional heater control device. In the figure a heater 1p is power-supplied from an A.C power source 18p through a bidirectional thyristor 2p connected in series with the heater. The thyristor 2p is gated by a switching signal from a control circuit to which a temperature signal of a sensor 6p is input, the control circuit comprises a comparator, a zero-crossing detector and a variable standard voltage circuit which provides a standard voltage corresponding to a predetermined value of temperature. The comparator compares a temperature signal from the temperature sensor 6p with the standard voltage, and outputs a trigger signal to the gate terminal of the thyristor 2p. On the other hand the zero-crossing detector is a circuit device which produces a pulse signal each time the instantaneous absolute value of the A.C power source voltage become smaller than a certain definite value. A diode 17p and capacitor 14p constitute a D.C power source to be supplied to the temperature sensor 6p and the control circuit. The control circuit, as a whole is devised so as to put the comparator into action only for the duration of the pulse signal produced by the zero-crossing detector. The comparator thus outputs to the thyristor 2p a trigger pulse with the same width as that of the pulse signal produced by the zero-crossing detector, only while the temperature is lower than the predetermined temperature value. Therefore, the heater 1p is energized with the thyristor 2p made conducting until the temperature of an object to be heated rises up to the predetermined value of temperature. However, such a circuit construction has a disadvantage that, in case the heater and the temperature sensor must be set remote from the main part of the device, the device needs two pairs of leading wires: one pair is for the heater and the other for the temperature sensor. A multiplicity of leading wires is apt to make a source of trouble due to the breakage of a wire. Particularly the breakage of the sensor wire may cause an uncontrolled running of the heater.